1. Field of the Invention
The present invention relates to a method of enhancing a border part included in an image of an original in an apparatus for scanning the image of the original and reading image data thereof, such as a process scanner (for a color or black-and-white image) or a facsimile.
2. Description of the Prior Art
As is well known in the art, various method have been proposed in order to enhance border parts included in images of originals read in a process scanner or the like.
According to the most basic method within these, a read signal (sharp signal) S from a pixel P (hereinafter referred to as "noted pixel"), which is a current object of reading, and an average read signal (unsharp signal) U from a region R around the noted pixel P are extracted first. Then the difference therebetween, i.e., a detailed original signal (S-U) is multiplied by a prescribed coefficient K, to regard the product as a detailed signal: EQU D.sub.T =K(S-U) (1)
Thereafter the sharp signal S is added with the detailed signal D.sub.T as follows: ##EQU1## This signal D is regarded as pixel data of the noted pixel P, thereby to perform border enhancement.
In a more advanced method proposed for border enhancement, a system of extracting an unsharp signal U is devised. FIGS. 2B and 2C illustrate such examples, while FIG. 2A shows the aforementioned basic method, i.e., a general unsharp masking (USM) method for reference.
Referring to FIG. 2A, an unsharp signal U is obtained by simply averaging read signals from a neighbourhood region R with uniform weight distribution W.sub.a. Referring to FIG. 2B showing the case of differential masking (DM), on the other hand, the weighted mean of read signals from a neighbourhood region R is obtained with weight distribution W.sub.b increased toward a noted pixel P and reduced toward peripheral parts, so that the same is regarded as an unsharp signal U. Referring to FIG. 2C, further, differential masking is digitally performed with digital weight distribution W.sub.c. The matrix lattice in FIG. 2C shows pixel array in the digitalized image data, and the neighbourhood region R is in the form of a rectangle.
These prior art examples are described in Japanese Patent Laying-Open Gazette No. 141871/1984 and Japanese Patent Publication Gazettes Nos. 27067/1964 and 24581/1964 respectively.
In each of the aforementioned methods, however, density difference is inevitably enhanced in proportion to difference between the sharp signal S and the unsharp signal U, regardless of the cause for such difference. Thus, strong enhancement may be erroneously performed even if there is no need for enhancement or only slight enhancement is required in practice.
For example, an original itself may have a rough surface or turbulence such as a pinhole or dirt (hereinafter referred to as "spot defect of original") may occur on the original to cause concentration of density change in a narrow area. If black dirt D.sub.st of about one pixel size is adhered to a white region of an original as shown in FIG. 3A, for example, a sharp signal S.sub.a indicates a low level (high density) when a pixel P in the position of this dirt D.sub.st is noted by scan reading while an unsharp signal U.sub.a obtained by averaging read signals from a wide region R including the pixel P indicates a high level (low density). Thus, strong enhancement is inevitably performed although there is no need to enhance the image of this dirt D.sub.st.
Further, such useless density difference enhancement may be performed at a degree higher than that of enhancement originally required in an image border part. For example, when a noted pixel P is close to a continuous borderline (visible outline) E shown in FIG. 3B, an unsharp signal U.sub.b is obtained as an average value of read signals of a high density region R.sub.S and a low density region R.sub.H which are adjacent to each other along the borderline E, and difference between the same and a sharp signal S.sub.b is not increased as the example of the dirt D.sub.st shown in FIG. 3A.
According to the conventional border enhancement method, as hereinabove described, not only density difference is enhanced in a spot defect part of an original but variable density difference of such defect part is inevitably enhanced at a stronger degree as compared with a continuous border part which must be enhanced.